There are known optical devices and methods which provide optical wave mixing and oscillators in photorefractive media. Relevant patent documents and other publications include:
(1) U.S. Pat. No. 4,529,273 to Cronin-Golomb, et al. issued July 16, 1985;
(2) "Theory and Application of 4 Wave Mixing in Photorefractive Media", M. Cronin-Golomb, B. Fischer, J. O. White and A. Yariv, IEEE J. Quantum Electronics QE-20, 12 (1984);
(3) "Spatial Light Modulation and Filtering Effects in Photorefractive Wave Mixing", B. Fischer, S. Weiss and S. Sternklar, App. Phys. Lett. 50, accepted for publication, December 1986;
(4) "Beam Coupling and Locking of Lasers using Photorefractive 4 Wave Mixing", S. Sternklar, S. Weiss, M. Segev and B. Fischer, Opt. Lett. 11, 528 (1986), made the subject of commonly-owned Israel patent application No. 79581, filed July 31, 1986 and incorporated herein by reference;
(5) "Optical Apparatus and Method for Light Beam Steering and Spatial Modulation", B. Fischer and S. Sternklar, Israel patent application No. 81723, filed Mar. 1, 1987, commonly-owned by the owners of the present application, and incorporated herein by reference;
(6) "Double Phase Conjugate Mirror: Analysis, Demonstration and Applications", S. Weiss, S. Sternklar and B. Fischer, Opt. Lett. February 1987;
(7) "Electro-Optic and Acousto-Optic Scanning and Deflection", M. Gottlieb, C. L. M. Ireland and J. M. Ley, Optical Engineering, Vol. 3 (1983), Marcel Dekker, Inc. New York and Basel;
(8) "Opto-Optical Light Deflection", G. T. Sincerbox and G. Roosen, Appl. Optics 22, 690 (1983); and
(9) "Dynamic Beam Deflection Using Photorefractive Gratings in BSO Crystals", G. Pauliat, J. P. Herriau, G. Roosen and J. P. Huignard, J. Opt. Soc. Am. B3, 306 (1986).
In addition, the technical literature, such as reference (1), has discussed the four-wave mixing process occurring in general photorefractive media, with particular reference to several types of phase conjugate mirrors. Various photorefractive oscillators were also described. Among them were various passive phase conjugate mirror (PPCM) arrangements.
The prior art relating to the operation of wave mixing oscillators also requires a single color beam from a single laser source common to all thhe mixing beams and also requoires external or internal (on the crystal surface) mirrors to create reflections for feedback to the photorefractive media. The use of a plurality of laser beam sources for coupling through the photorefractive media was considered impossible to achieve because of mutual coherency requirements. Also, prior art PPCM's were pumped by a single incident laser beam, as disclosed in background reference (1).
Most electronic and electro-optic techniques for light beam steering are based on the use of tunable volume gratings via acousto-optic or photorefractive effects, as disclosed respectively in references (7) and (8). These methods share the same basic limitation posed by the Bragg condition, namely, the existence of an upper limit on the deflecting range. Several techniques have been suggested to improve this limit, among these being the use of a pre-correction grating as discussed in reference (8).
There are many and varied laser applications which require phase locking of laser sources. The prior art teaches several methods for achieving this, such as injection locking for gas lasers and other external locking techniques, each of which requires careful optical alignment. In addition, these methods are adversely affected by aberrations in the laser cavity due to non-ideal optics or light guiding, thermal and other light path distortion effects which lower the overall coupling efficiency.